Investments in Partially Owned Entities (Alexander's Inc.) (Details) - Alexanders Inc		1 Months Ended	12 Months Ended
	Apr. 05, 2018 USD ($)	Nov. 30, 2012 USD ($)	Dec. 31, 2018 USD ($) $ / shares $ / ft² shares	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Equity Method Investments And Income From Equity Method Investments
Investment owned, balance, shares | shares			1,654,068
Equity method ownership percentage			32.40%
Amount Alexander owed			$ 708,000	$ 2,490,000
Closing share price (in dollars per share) | $ / shares			$ 304.74
Equity method investment, quoted market value			$ 504,061,000
Excess of investee's fair value over carrying amount			396,078,000
Excess of investee's carrying amount over equity in net assets			39,046,000
Additional real estate tax expense			$ 7,708,000
Managment Agreement
Property management fee, percent fee, description			We receive an annual fee for managing Alexander’s and all of its properties equal to the sum of (i) $2,800,000, (ii) 2% of the gross revenue from the Rego Park II Shopping Center, (iii) $0.50 per square foot of the tenant-occupied office and retail space at 731 Lexington Avenue, and (iv) $315,000, escalating at 3% per annum, for managing the common area of 731 Lexington Avenue.
Management fee income			$ 2,800,000
Development fee percent			6.00%
Leasing Agreement
Leasing services fee description			leasing services for a fee of 3% of rent for the first ten years of a lease term, 2% of rent for the eleventh through twentieth year of a lease term and 1% of rent for the twenty-first through thirtieth year of a lease term, subject to the payment of rents by Alexander’s tenants.  In the event third-party real estate brokers are used, our fee increases by 1% and we are responsible for the fees to the third-parties.
Lease fee percentage of rent one to ten years			3.00%
Lease fee percentage of rent eleven to twenty years			2.00%
Lease fee percentage of rent twenty first to thirty years			1.00%
Percentage increase lease fee if broker used			1.00%
Percentage commissions on sale of assets under fifty million			3.00%
Asset sale commission threshold			$ 50,000,000
Percentage commissions on sale of assets over fifty million			1.00%
Building Maintenance Service ("BMS")
Other Agreements
Fee Income recognized by equity method investees for building maintenance services			$ 2,705,000	$ 2,678,000	$ 2,583,000
Kings Plaza Regional Shopping Center
Equity Method Investments And Income From Equity Method Investments
Real estate tax expense		$ 3,971,000
Additional real estate tax expense	$ 23,797,000
Lexington Avenue 731
Managment Agreement
Property management fee agreement price per square foot | $ / ft²			0.50
Rego Park 2 Property
Managment Agreement
Property management fee agreement percentage of income			2.00%
Common Area 731 Lexington Avenue
Managment Agreement
Management fee income			$ 315,000
Property management fee agreement escalation percentage of income			3.00%
Vornado Realty Trust
Equity Method Investments And Income From Equity Method Investments
Equity method ownership percentage			32.40%